J . Am. Chem. Soc. 1986, 108, 6986-6991
6986
Photolysis of (Cyclopentadieny1)- and (Pentamethylcyclopentadieny1)tricarbonylhydridometa1 Complexes of Tungsten and Molybdenum in Dihydrogen-Containing Matrices: Evidence for Adducts of Molecular Hydrogen Ray L. Sweany Conlribution from the Department of Chemistry, University of New Orleans, New Orleans. Louisiana 70148. Received March 6, 1986
Abstract: When HMCP(CO)~and HMCp’(CO), (M = Mo and W; Cp = $-CsH5; and Cp’ = qs-C5(CH3),) are photolyzed with a low-pressure mercury lamp in dihydrogen-containing matrices, new bands appear in the carbonyl region of the infrared spectrum which are assignable to simple adducts of molecular hydrogen. These bands have been assigned to cisoid and transoid isomers of HM(H2)Cp(C0), and HM(H,)Cp’(CO),. The molybdenum complexes can be shown to form from H M o C ~ ( C O ) ~ or H M o C ~ ’ ( C O )with ~ radiation of X > 400 nm. The dihydrogen complex is destroyed by using radiation of X C 400 nm. The principal product of this latter process is H M o C ~ ( C O or ) ~HMoCp’(CO),. Thus, all the steps leading to the formation of the adducts from the parent HMoCp(CO), are reversible. The tungsten dihydrogen adducts form from HWCp(CO), in an analogous fashion via the intermediacy of HWCp(CO),. However, the two isomers of HW(H,)CP(CO)~do not form HWCP(CO)~upon subsequent photolysis. Rather, a new species is formed which gives spectral data consistent with H,WCpCO, in which the hydrogen is oxidatively added.
for such an arrangement, being free of additional atoms which might cause crowding. It is surprising therefore that the first complexes of this sort that were characterized were of the more encumbered carbon-hydrogen bonds. The scarcity of similar complexes of dihydrogen is due, in part, to the strengths of metal-hydrogen bonds; many attempts at forming dihydrogen adducts will result in dihydrides. Second, it is experimentally more difficult to maintain high concentrations of hydrogen to assure that a complex having temporarily lost dihydrogen will reform an adduct before the coordinatively unsaturated intermediate is scavenged. The matrix isolation technique offers several advantages for the study of complexes of dihydrogen. Large concentrations of dihydrogen can be doped into a matrix so that every metal complex is entrapped with at least one molecule of hydrogen. Usually, the metal complex can be activated by photodissociation of a ligand. This forms an intermediate which can undergo very few types of reactions. It can react with hydrogen and with the ligand which was expelled or undergo an intramolecular transformation, leading perhaps to an unreactive product. Several complexes of hydrogen and the group 6 metal pentacarbonyls have been isolated.2 Presuming that oxidative addition might occur for a more electron rich group 6 metal, several complexes of the general formula of HMCp(CO)* have been formed in matrices which contain dihydrogen. A portion of this work has been reported in a recent communication.
Recently, a great deal of attention has been paid to stable complexes of a metal and a ligand in which the ligand acts as a donor with use of electron pair that populated a ligand 0 bond. The resultant three-center interaction has been labeled agostic when a carbon-hydrogen bond forms an adduct with a metal.’ More recently, similar interactions have been characterized for dihydrogen.2-6 Presumably, similarly configured complexes are intermediates during oxidative addition to a metal of a variety of substrates.’ Whether a system will undergo oxidative addition depends on the energies of the newly created bonds to the metal relative to the strength of the 0 bond that is broken in the ligand.8 Thus, it is expected that a third row transition metal is more likely to oxidatively add a ligand because the new metal-ligand bond energies are almost always greater for such a metal as compared to the lighter congener^.^ When oxidative addition is not favored, there still exists the potential for complexation. This latter type of interaction will be sensitive to crowding caused by the other ligands on the metal and by groups which are bonded to the ligating atoms.I0 Dihydrogen would appear to be ideally suited (1) Crabtree, R. H.; Holt, E. M.; Lavin, M.; Morehouse, S. M. Inorg. Chem. 1985, 24, 1986-1992. Bennett, M. A.; McMahon, I. J.; Pelling, S.; Robertson, G. B.; Wickramasinghe, W. A. Organometallics 1985.4, 754-761 and references cited therein. (2) Sweany, R. L. J . Am. Che n. SOC.1985, 107, 2374-2379. (3) Upmacis, R. K.; Gadd, G. E.; Poliakoff, M.; Simpson, M. B.; Turner, J. J.; Whyman, R.; Simpson, A. F. J . Chem. Soc., Chem. Commun. 1985, 27-30. (4) Kubas, G. J.; Ryan, R. R.; Swanson, B. I.; Vergamini, P. J.; Wasserman, H. J. J . Am. Chem. SOC.1984, 106,451-452. ( 5 ) Crabtree, R. H.; Lavin, M. J . Chem. Soc., Chem. Commun. 1985,
Experimental Details Matrices were formed on the cold-end of a Displex refrigerator; descriptions of the apparatus and procedures have already been published.I2 HMoCp(CO), was prepared from M o ( C O ) ~ Ior, ~ alternatively, [ M O C ~ ( C O was ) ~ ] cracked ~ with a sodium amalgum to give NaMoCp(CO), which was then treated according to the former procedure. Matrices were formed by subliming the hydride at 0 OC into the flowing matrix gas mixture. Similar conditions were used by Rest to obtain good isolation as judged by the widths of infrared absorptions of the carbonyl ligands.I4 HMoC~’(CO)~ was prepared from C7HsMo(CO)3and HC5(CH,),.1S The molecule was enriched with I3COby dissolving the
794-795.
(6) Morris, R. H.; Sawyer, J. F.; Shiralian, M.; Zubkowski, J. D. J . Am. Chem. SOC.1985, 107,5581-5582. Gadd, G. E.; Upmacis, R. K.; Poliakoff, M.; Turner, J. J. J . Am. Chem. SOC.1986,108,2547-2552. Conroy-Lewis, F. M.; Simpson, S. J. J . Chem. Soc., Chem. Commun. 1986, 506-507. Ozin, G. A.; Garcia-Prieto, J. J . Am. Chem. Soc. 1986, 108, 3099-3100. Crabtree, R. H.; Hamilton, D. G.J . Am. Chem. SOC.1986, 108, 3124-3125. (7) Saillard, J. Y . ; Hoffmann, R. J . Am. Chem. SOC.1984, 106, 2006-2026. Sevin, A.; Chaquin, P. Nouu. J . Chem. 1983, 23, 855-863. Stoutland, P. 0.;Bergman, R. G. J. Am. Chem. SOC.1985,107,4581-4582. (8) Abis, L.; Sen, A,; Halpern, J. J . Am. Chem. SOC. 1978, 100, 2915-2916. Halpern, J. Inorg. Chim. Acra 1985, 100, 41-48. (9) Skinner, H.A.; Connor, J. A. Pure Appl. Chem. 1985, 57, 79-88. (10) Tsou, T.; Loots, M.; Halpern, J. J . Am. Chem. SOC.1982, 204, 623-624.
0002-7863 /86/1508-6986SO1 .Sol0 I
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(11) Sweany, R. L. Organometallics 1986, 4, 387-388. (12) See ref 2 and references cited therein. (13) Fischer, E.0. Inorg. Synth. 1963, 7, 136-139. (14) Mahmoud, K. A.; Rest, A. J.; Alt, H. G. J. Chem. Soc.,Dalton Tram. 1984. 187-197.
0 1986 American Chemical Societv , -
J . Am. Chem. SOC.,Vol. 108, No. 22, 1986 6981
Photolysis of HMCp(CO), carbon-12 isotopomer in hexane. The solution was photolyzed in a quartz vessel with a medium-pressuremercury lamp under an atmosphere of I3CO. Matrices were formed by direct sublimation from the solvent-free residue. The product was also analyzed by mass spectroscopy. From this analysis, it was shown that other materials formed by the photolysis were probably not volatilized under the conditions of the matrix experiment. The most reliable measure of the extent of enrichment comes from a comparison of the relative intensity of the infrared absorptions of free carbon monoxide which was produced by photolyzing the enriched sample. HMoCp’(CO), was sublimed at room temperature into flowing argon/hydrogen in order to form matrices with adequate isolation. HWCp(CO), was produced in the same manner as HMoCp(CO),.” A sublimation temperature of 10 OC gave matrices which appeared to be adequately dilute. HWCp’(CO), was producedI6 from W(CH3CN)3(CO)317and HC5(CH3)S. The complex was sublimed at 35 ‘C. Infrared spectra were obtained on a Beckman 4260 infrared spectrometer. In the later phases of this research, the spectra were digitized by a Dual Systems 12-bit analog-to-digitalconverter and collected by a Zenith 2-100 computer. The data rate was synchronized with the spectrometer drive and, most frequently, data were obtained every 0.2 cm-I. Data obtained in this fashion were summed over replicate scans in order to enhance the signal to noise. In one instance, the resultant spectrum was fit to a simulated spectrum with a curve fitting routine written by Pitha and Jones of the National Research Council of Canada. In another instance, spectra which had been converted to absorbance units were subtracted from each other. Because the spectra were taken at quite different times, the spectrometer wavelength marker was included in the data set so that absolute frequency position of the data could be ascertained. Infrared frequencies in the carbonyl region were corrected to vacuum conditions with spectra of DCI and DBr.’* Frequencies in that region are accurate to i l . 0 cm-I. Absorptions in other regions of the spectrum are accurate to i 3 . 0 cm-l.
Results and Discussion When inert gas matrices of H M C P ( C O ) ~are irradiated with ultraviolet light, two processes are observed, CO loss and H-M hom01ysis.l~ The dominant process yields HMCp(CO), and free CO. They usually fail to recombine as long as the matrix is protected from visible or ultraviolet photons. Upon irradiation the 16 electron fragment becomes reactive, either recombining with the nearby C O or combining with nearby donor molecules such as N, or ethylene. Also, ultraviolet irradiation of HMCp(CO), yields the radical products, H and MCp(CO),. These products have only been observed in CO matrices in which the C O loss process is surpressed and the reactive hydrogen atom is scavenged by C O to form HCO. The products of this latter process recombine much less efficiently than those formed by the C O loss process, presumably because the hydrogen atom is propelled out of the immediate vicinity of M C P ( C O ) ~ . When HMCp(CO), is irradiated in the presence of H1, new bands are observed which are assigned to adducts of molecular hydrogen. The chemistry of the tungsten complexes is sufficiently different from that of molybdenum that the discussion will be broken into two parts. HMoC~(CO)~. The infrared spectra of HMoCp(CO), (Ia) or H M o C ~ ’ ( C O )(Ib) ~ in hydrogen-containing matrices are quite similar. For a molecule of Cssymmetry one expects three infrared allowed transitions involving the carbonyls. Two of the modes are nearly coincident; the pair appears at 1951 and 1939 cm-I for HMoCp(CO), and H M o C ~ ’ ( C O ) respectively. ~, The bands are not symmetric, suggesting that the absorption profile is the envelope of two modes. However, one cannot unambiguously assign the splitting to the presence of two fundamentals because of the possibility of site splitting in the solid state. Rest and co-workers have successfully fit the carbonyl spectrum of HMCp(CO), in matrices using carbon-13 data, assuming the band (15) Nolan, S. P.; Hoff, C. D.; Landrum, J. T. J. Organomet. Chem. 1985, 282, 357-362. (16) King, R. B.; Fronzaglio, A. Inorg. Chem. 1966, 5, 1837-1846. (17) Tate, D. P.; Knipple, W . R.; Augl, J. M. Inorg. Chem. 1962, I , 433-434. (18) International Union of Pure and Applied Chemistry, Commission on Molecular Structure and Spectroscopy: Table of Wauenumbers for rhe Calibration of Infra-red Spectrometers; Butterworths: Washington, D.C., 1961.
Table I. Positions of the Carbonyl Bands of HMoCp(CO), and Its Derivatives in Ar/H2 and Ar/D2 Matrices (in cm-l)
I
I1 (H2) 11 (D2) 111 (H2) 111 (D2)
IV
2034.4 1950.0 2000.6 1930.6 2000.2 1928.8 1988.9 1921.4 1987.1 1919.2 1975.0 1895.8
2033.4 1949.6 2000.4 1930.2 2000.4 1929.3 1988.6 1920.7 1988.1 1918.7 1973.6 1894.8
2021.2 1938.7 1984.1 1913.2 1983.6 1912.0 1973.8 1904.2 1972.8 1903.7 1956.8 1876.3
2020.6 1938.6 1983.9 1913.3 1984.4 1912.2 1973.6 1905.4 1973.3 1903.1 1957.4 1876.2
Table 11. Positions of the Carbonyl Bands of HWCp(C0,) and Its Derivatives in Ar/H2 and Ar/D2 Matrices (in cm-l)
HWCp(CO), V VI (H2)
VI (Dd VI1 (H2) VI1 (D2)
VI11 (H2)
VI11 (D2) IX
2032.0 1943.0 2016.7 1947“ 2016.0 1947” 2034” 1975.7 2032‘ 1973.6 2055.2 2055.6 1968 1884.6
DWCp(C0)S HWCp’(CO)3 2032.0 1943.2 2016.3 1947“ 2016.9 1947” 2034“ 1974.8 2032’ 1972.6 2054.2 2055.5 1967.0 1884.0
2019.0 1930.8 2003.1 1931“ 2003.4 1931“ 20 19“ 1959.7 20 19“ 1957.7 2045.1 2046.3 1949.6 1865.8 ... .
“Approximate position, due to near superposition of band with that of parent. at 1951 cm-l was in fact the envelope of two fundamental^.'^ A similar result attains for the spectrum of Ib, the results of which are summarized in Table 111. When HMoCp(CO), or H M o C ~ ’ ( C O )is~photolyzed with a low-pressure mercury lamp in the presence of H,, two sets of new bands appear in the carbonyl region which correlate with the presence of hydrogen in the matrix. Each set consists of two bands. The frequencies are tabulated in Table I. The species which are responsible for these new absorptions shall be referred to as IIa and IIb, and IIIa and IIIb.I9 The bands due to I1 and 111 grow at approximately the same rate as a function of photolysis time and hydrogen abundance in the matrix, suggesting that they have similar compositions. When the bands due to HMoCp(CO), become attenuated in response to exposure to the full irradiance of the Nernst glower, bands due to I1 and 111 grow in. This suggests that I1 and 111 are formed from HMoCp(CO), and H2. The parent molecule (I) has been referred to as a piano stool molecule; the cyclopentadienyl ring functions as the seat while the hydride and three carbonyl ligands splay out in the opposite direction as if they were legs. One can view I1 and 111 as simple substitution products of the parent, with H, functioning as a two-electron donor analogous to what is observed in H,W(CO)3(PR3)2.4There is the possibility of two isomers of such a compound, one with its two remaining C O ligands in a transoid orientation and the other with its ligands in a cisoid orientation. Although the bands of I1 and 111 are not completely resolved from each other, it is clear that the intensities of the two bands of I1 are fairly dissimilar, whereas the intensities of the two bands of I11 are nearly the same. Fitting the spectra with a product function of Lorentzian and Gaussian line shapes yields an estimate of the relative intensities of the two features. The ratio of intensities (19) Throughout this report, the species which arise from HMCP(CO)~ shall be referred to with an a, whereas those species which arise from HMCP’(CO)~will be referred to with a b. The letter will be dropped when the two types of species are being referred to collectively.
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6988 J . Am. Chem. SOC.,Vol. 108, No. 22, 1986 __
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Table 111. Force Constants of H M C P ( C O ) ~and Its Derivatives ImdvnlA) HMoCp'(CO)3 Ib IIb IlIb IVb' HWCP(CO)S Vd VIa VIIa VIIIa
